It's dirt simple - I actually converted a "world population" widget into what I wanted, hence some of the funny names. "Startpop" is your starting bank account. "Poprate" is your burn rate per second. "Startdatepop" is the day you start the counter.

For example, in my script I budgeted $1000/month, which works out to $0.00038/s

Like I said, it's pretty crude. You have to routinely update both your balance and the start-date to keep it accurate...but I've found just the idea of a ticking balance is more powerful than what the number actually is.